Planned, I think. There was a reason they stopped doing the JoJ - because the lore in those was stagnant and nobody cared about it. Then, suddenly, they revamped several characters's backstories, relaunched Twisted Treeline and the Shadow Isles, etc etc etc. There was a big upheaval around that time.

Her Mother was the Fist of Shadow before her, a role within the Kinkou, not Zed's clan. This means her mother probably left Zed shortly before Akali was born to join the Kinkou, probably due to the raid on Shen's master's temple.

Or an alternate theory: Akali's mother might have been lovers with Zed, or at least a protegee. But shortly after her pregnancy, she had a change of mind, and didn't want her child to follow the same path as her. She regretted the Path of Shadows, and defected.

She left Zed's order during this time and sought after the Kinkou Order for protection. It is reasonable to assume that she hid inside the temple at some point.

Zed finds out his favorite student has defected to the Kinkou, and decides to assault the temple, not only to seek revenge, but also to find her.

You've read Zed's lore, right? What the hell do you think happens at this point:

The old man implored Zed to enter the temple, destroy the box, and lead his followers to balance. The dark ninja followed the master inside. Moments later, the assembled ninjas heard Zed cry out in pain. Mysteriously, he emerged unscathed, and threw the severed head of the master at Shen's feet. Screaming in rage, Zed commanded his followers to slaughter the master's students and seize the box.

Shen's father banished Zed in order to let him grow stronger so he could steal his body. He's basically a parasite, attempting to destroy the institutions of Ionia from the inside.

There's also the possibility that LeBlanc is actually Shen's father (and Zed by extension). Plus Syndra. Basically everyone, including Karma, is LeBlanc. There's no lore justification for Karma=LeBlanc, but look at their skills. Slow with delay into root, massive nukes in small AoE's, and an ultimate that isn't really an ultimate? Sound like anyone we know?

So:
LeBlanc is a man;
LeBlanc is everyone;
Shen and Akali are brother and sister;
Shen and Akali are really LeBlanc;
LeBlanc is her own father, brother and sister. (And grandfather too, just for the futurama reference)

So what we're getting at with this is that Shen/Zed are actually one person that has dissociative identity disorder.

So he killed his father, and then found his father's head after the transition and vowed revenge on whoever did it.

Which is him.

Thus, Shen and Zed will never have a final epic ninja battle because of this. Which is quite sad because that would be a fantastic conclusion.

If Riot ever starts getting rid of champions to make ways for new ones, I fully expect something like this to happen. Ninjas v Ninjas. Garen v Darius. Nasus v Renekton. Winner stays, loser dies. Or, if they dislike the whole 'dying' thing, they could just have their ability to be summoned revoked. So they can never fight in the League of Legends again.

No, Shen and Zed trained under Shen's dad. If you read the lore, Zed was an orphan or something. There was a forbidden-no-jutsu in a box that Zed peeked in and gained shadow powers. With the new powers Zed beat Shen in training. But was exiled for using forbidden jutsus.

He comes back later to the temple with his army of students. Shen's dad invites him back to destroy the box however (I'm guessing here) the box's influence was too strong and he killed Shen's dad instead. He then throws the dad's head at Shen's feet and has an all out ninja war with Shen.

In the end, Shen escaped with the remaining students and Zed took the temple.

This gives me an all new satisfying feeling when I kill Akali now, his E can straight up fuck akali when she's hiding in her shroud. Good work on the lore. I always wish they just came out with it all at once though.